Display devices become more and more important in electronic equipment and customer devices. Therefore, the technical development also focuses on processes and devices of illumination, image generation, and projection. It is a well-known problem that illumination devices and respective light sources do not only generate light in the respective channels with respective profiles and distributions but also tend to inherently produce some parasitic noise. These noise are called speckles in the field of laser light sources.
Lens arrays—often in the form fly-eye lenses or integrator plates—are commonly used in projection systems in order to uniformly illuminate an image modulator.
Laser projectors today are commonly realised by using one single laser beam for each colour or colour channel R, G, B which scans line by line over the screen (2D scanning projector). The image is generated on the screen by modulating the beam intensity synchronously with the line frequency. For the horizontal and vertical movement of the beam two scanning mirrors are used. Such kind of laser projectors with a single scanning beam are a potential safety risk for viewers, because in case of a malfunction of the scanning mirrors a static laser beam of high intensity is projected to the screen.
Another approach to use laser light in projectors is realized by the GLV (Grating Light Valve) technology. In such devices the laser beam is expanded to form a vertical line, which is projected to a one-dimensional image modulator (GLV chip). This GLV chip modulates the intensity of the reflected light to generate an vertical image line. The reflected light is projected to the screen and—by use of one horizontal scanning mirror—it is scanned over the screen. A full image is generated by changing the image content of the GLV chip synchronously with the horizontal scanning mirror.
Laser speckle is commonly reduced by the presence of a moving random diffuser or random phase retarder in the optical path, at a point where the laser is focused before the image formation device or at a plane where the image is formed in the optical systems: (see: Trisnadi in Proc SPIE 4657, 2002).
Today's projectors (data- and video-projectors) commonly use arc-lamps as light source. These lamps have several drawbacks: As these lamps emit white light, the light must be split into the primary colours (R, G, B) by the use of filters, which add size, weight and cost to the projection system. Moreover, light, which is not within the spectral range of the primary colours, is lost, thereby reducing the efficiency of the system.
Another drawback is the emission of IR- and UV-radiation. The IR-radiation produces heat and the UV-radiation causes degradation of organic components during aging.
The glass bulb containing the electrodes of the arc lamp has an operating temperature close to 1000° C. and consequently has to be controlled and cooled by a forced airflow, which causes annoying noise.
Light emitted by high-pressure arc lamps is unpolarized, which is a drawback when used in combination with polarizing image modulators like LCD (Liquid Crystal Display) or LCoS (Liquid Crystal on Silicon).
An important quantity in the design of projectors is the étendue (optical extent). In a simplified description the étendue is defined by the product of the cross-sectional area of a light beam with the beam divergence angle of that beam at a certain position. If a light beam is modified by an optical element, the étendue is preserved if the optical element was well corrected or the étendue is increased if the optical element was not well corrected. It is impossible to decrease the étendue of a beam by any kind of optical transformation, except the case where part of the beam is simply cut and light is lost. In other words, the image luminance can never exceed the source luminance. This has consequences for the design of projectors, as the optical elements in a projector have a limited size (this limits the beam cross section) and/or a limited acceptance angle (this limits the beam divergence angle). For example the projection lens has a certain f/# which limits the beam divergence angle. Moreover, the image modulators are limited by size (cost) and beam divergence angle: The bigger the beam divergence angle of a light beam the lower is the contrast of the modulated image. As a result, the étendue of the light source must be smaller than the étendue of the most limiting optical element in a projector. The étendue of a light source should be as small as possible in order to achieve compact projectors with good image quality (contrast, brightness).
An ideal projection lamp with zero étendue would be on one hand a point source (zero cross section, but high beam divergence) or on the other hand a collimated beam without divergence (any cross section but zero beam divergence). In reality the étendue of arc lamps is determined by the size of the arc. The minimal possible arc length is restricted due to the thermal stress of the electrodes, which strongly influences the lifetime of the lamp. As a compromise, commonly used arc lamps today have arc lengths slightly below 1 mm and lifetimes between 2000 and 6000 hours (dependent on power).
LEDs or light emitting diodes as light source in a projector can overcome many of the above-mentioned drawbacks of arc lamps.
LEDs
                emit inherently light in the wavelength range of primary colours (R, G or B).        do not emit UV- or IR radiation.        can be cooled passively or at least with a low air flow without generating fan noise.        have potentially a long lifetime.        can be driven in a pulsed mode for sequential color generation.        
But there a still some drawbacks.                LEDs emit unpolarized light, which is a drawback if used in combination with polarizing image modulators (LCD, LCoS).        LEDs have a significant étendue (emitting light with a high divergence angle, usually like a Lambertian emitter).        
Laser light sources are very close to an ideal projection light source, as they can solve all of the above mentioned drawbacks of arc lamps and LED.
Laser Light Sources
                emit inherently monochromatic light (R, G or B).        do not emit UV- or IR radiation.        can be cooled passively or at least with a slow air flow without generating fan noise.        have potentially a long lifetime.        can be driven in a pulsed mode for sequential color generation.        emit inherently polarized light        have extremely low étendue (close to zero).        
Laser projectors today are commonly realised by using one single laser beam for each color (R, G, B) which scans line by line over the screen (2D scanning projector). The image is generated on the screen by modulating the beam intensity synchronously with the line frequency. For the horizontal and vertical movement of the beam two scanning mirrors are used. Such kind of laser projectors with a single scanning beam are a potential safety risk for viewers, because in case of a malfunction of the scanning mirrors a static laser beam of high intensity is projected to the screen.
Another approach to use laser light in projectors is realized by the GLV (Grating Light Valve) technology. In such devices the laser beam is expanded to form a vertical line which is projected to a 1-dimensional image modulator (GLV chip). This GLV chip modulates the intensity of the reflected light to generate an vertical image line. The reflected light is projected to the screen and—by use of one horizontal scanning mirror—it is scanned over the screen. A full image is generated by changing the image content of the GLV chip synchronously with the horizontal scanning mirror.
A third method to use lasers in projectors is to illuminate image modulating panels like LCD (Liquid Crystal Display) or LCoS (Liquid Crystal on Silicon) or MEMS (Micro-electromechanical Systems). In this case the image is completely generated by the image modulating panel and the laser light just illuminates the (rectangular) aperture of the panel. This is very similar to any standard projector which uses arc lamps for illumination. Several groups have proposed such kind of laser projectors but today no such product is on the market. DOEs (Diffractive Optical Elements) or refractive optical elements like microlens arrays are used for beam shaping to illuminate the rectangular aperture of the panel.
But due to the coherence and low étendue of laser beams another phenomenon is observed when projecting laser light to a screen: Laser light is scattered on the diffusing screen and a parasitic noise called speckle is observed. Laser speckle is commonly reduced by the presence of a moving random diffuser in the optical path, at a point where the laser is focused before the image formation device or at a plane where the image is formed in the optical systems. (see Trisnadi in Proc SPIE 4657, 2002). Another proposed method to reduce speckles is the use of acousto-optic diffuser plates (G. Bastian, GMM Workshop Mikrooptik in Karlsruhe, 3/4 Feb. 2005).
The quantity to describe speckle is the speckle contrast c, which is defined by c=σ/I. (see J. W. Goodman in J. Opt. Soc. Am, Vol. 66, No. 11, November 1976). Here σ is the standard deviation of the intensity distribution of the pattern and I is the mean intensity of the pattern. In the worst case c equals 1, which means maximum noise of the image. In the best case c equals 0, which means a perfectly smooth (σ=0) image.
The general idea to reduce speckle with moving or time-varying diffusers is to generate as many as possible different speckle patterns within the integration time of the detector (human eye). The superposition of N uncorrelated speckle patterns within the integration time gives a smoother image with a speckle contrast reduced by factor √N (see J. W. Goodman in J. Opt. Soc. Am. Vol. 66, No. 11, November 1976). Example: 625 uncorrelated speckle pattern during one image frame (assuming 50 Hz) are required in order to reduce the speckle contrast by a factor 25=√625. This means 625 different and uncorrelated patterns must be generated within 20 ms.    i) Now assuming a 2D scanning laser projector with 50 Hz image frame rate and HDTV resolution (1920×1080 Pixel), the laser beam needs 9.6 ns to scan over one pixel. As this occurs only once during the frame period of 20 ms, the 625 different pattern must be generated within 9.6 ns when the beam is passing the pixel. This translates to a pattern frequency of 64.8 GHz.    ii) In the case of a 1D scanning laser (GLV), the laser beam needs approx. 10 μs to scan over one pixel-line. With 625 different pattern during this period this translates to a pattern frequency of 60 MHz.    iii) When illuminating an image modulator (LCD, LCoS, MEMS), then the laser light is continuously illuminating each pixel (except in the case where the laser is pulsed). In that case 625 different patterns must be generated during the period of 20 ms which translates into a pattern frequency of 31.25 kHz.
The conclusion from i) to iii) is, that speckle reduction by moving or changing diffusers is easiest in case iii).
Another method to reduce the speckle contrast is to use several laser light sources in parallel, e.g. laser array, where the lasers are incoherent to each other. If using N uncorrelated lasers then the speckle contrast again is reduced by factor √N (see: J. W. Goodman in J. Opt. Soc. Am, Vol. 66, No. 11, November 1976).
In EP 1328128 a laser projection display system is disclosed, which includes a laser light source for emitting a light beam having a coherence length; a beam expander for expanding the light beam; a spatial light modulator; beam shaping optics for shaping the expanded laser beam to provide uniform illumination of the spatial light modulator, the beam shaping optics including a fly's eye integrator having an array of lenslets; a diffuser located in the light beam between the laser light source and the beam shaping optics; an electrically controllable de-speckling modulator for modifying the temporal and spatial phase of the light beam; and a projection lens for producing an image of the spatial light modulator on a distant screen. The electrically controllable de-speckling modulator comprises a bulk electro-optic substrate with a series of individual modulator sites that receive control signals to provide localized random phase changes to the incident light. By independently electrically addressing the modulator sites, each site in the electrically controllable de-speckling modulator can introduce phase delays in the light beam with respect to the neighbouring sites. A different voltage or voltage duration is applied to each site, thus producing a phase delay corresponding to as much as several waves of propagation in the light. Each modulator site has a delay region through which a beam of light encounters a time or phase delay in relation to the electric field applied between a top electrode and a bottom electrode, when a beam of light enters bulk electro-optic substrate through an input facet, after which it traverses the modulator, and exits through an output facet. The use of bottom and top electrodes on the bulk electro-optic crystal results in a rather elaborated and difficult production scheme in order to obtain a de-speckling modulator in two dimensions.